Conventionally, an optical pickup device has been used for recording an information signal to or reproducing a recorded information signal from an optical disc, a magneto-optical disc or the like. The optical pickup device of this type has an optical system 101 constituted as shown in FIG. 1. The optical system 101 has, in the order of the optical path, a light source 111 for emitting a light beam for scanning a signal recording area of an optical disc 104, a diffraction grating 112 for splitting the light beam emitted from the light source 111, a beam splitter 113 for splitting the light beam and return light from the optical disc 104, an aperture diaphragm 114 for stopping down the light beam to a predetermined numerical aperture NA, an objective lens 115 for condensing the light beam lo the optical disc 104, and a light receiving unit 116 for receiving the return light beam reflected from the optical disc 104, as shown in FIG. 1.
A semiconductor laser is used as the light source 111, which emits a laser beam. The diffraction grating 112 splits the light beam emitted from the light source 111 into three beams made up of zeroth-order light, plus-first-order light and minus-first-order light, in order to acquire a tracking error signal by using a so-called three-beam method. The beam splitter 113 has a half mirror 119 for reflecting the light beam emitted from the light source 111 and for transmitting the return light from the optical disc 104, and thus splits the light beam from the light source 111 and the return light beam.
Although not shown, the light receiving unit 116 has a main beam photodetector for receiving the zeroth-order light split from the return light beam by the diffraction grating 112, and a set of side beam photodetectors for receiving the plus-first-order light and the minus-first-order light split from the return light beam by the diffracting grating 112, respectively.
As a method for detecting a focusing error signal, a so-called astigmatism method is used in the optical system 101. Therefore, a main beam photodetector 121 is formed with a substantially rectangular light receiving surface for receiving the return light and has a split pattern including light receiving areas a2, b2, c2, d2 which are formed by quadrisecting the light receiving surface along a set of dividing lines passing through the center of the light receiving surface and orthogonal to each other, as shown in FIGS. 2A, 2B and 2C. Although not shown, the side beam photodetectors are arranged at positions to face each other with the main beam photodetector 121 provided between them.
In the forward path from the light source 111 to the optical disc 104 of the optical system 101, the optical components are arranged so that an image point as a conjugate point of an emission point of the light source 111 as an object point is situated on a recording surface 105 of the optical disc 104, as shown in FIG. 1
In the backward path from the optical disc 104 to the light receiving unit 116 of the optical system 101, the optical components are arranged so that an image point as a conjugate point of a point on the recording surface 105 of the optical disc 104 as an object point is situated on the light receiving surface of the main beam photodetector 121 of the light receiving unit 116.
Therefore, in the optical system 101, the emission point of the light source 111 and the point on the light receiving surface of the main beam photodetector 121 of the light receiving unit 116 are conjugate with each other.
A method of acquiring a focusing error signal from the light receiving areas a2, b2, c2, d2 of the above-described main beam photodetector 121 will now be described.
First, if the objective lens 115 is situated at an optimum position with respect to the recording surface 105 of the optical disc 104 and is in focus with respect to the recording surface 105 of the optical disc 104, that is, if the objective lens 105 is in an accurate focusing state, the shape of a beam spot on the light receiving surface of the main beam photodetector 121 is circular, as shown in FIG. 2B.
If the objective lens 115 is too close to the recording surface 105 of the optical disc 104, the objective lens 115 gets out of focus and the return light passing through the beam splitter 113 generates astigmatism, which causes the shape of a beam spot on the light receiving surface of the main beam photodetector 121 to be elliptical with its long axis extending into the light receiving areas a2 and c2, as shown in FIG. 2A.
Moreover, if the objective lens 115 is too far from the recording surface 105 of the optical disc 104, the objective lens 115 gets out of focus and the return light passing through the beam splitter 113 generates astigmatism, which causes the shape of a beam spot on the light receiving surface of the main beam photodetector 121 to be elliptical with its long axis extending into the light receiving areas b2 and d2, as shown in FIG. 2C. This elliptical shape has its long axis inclined by 90 degrees from the above-described shape of the beam spot shown in FIG. 2A.
When the return light outputs from the light receiving areas a2, b2, c2, d2 of the main beam photodetector 121 are expressed by Sa2, Sb2, Sc2, Sd2, a focusing error signal FE is calculated by the following equation (1).FE=(Sa2+Sc2)−(Sb2+Sd2)  (1)
Specifically, if the objective lens 115 is situated at the focusing position, that is, if the objective lens 115 is in the accurate focusing state, as shown in FIG. 2B, the focusing error signal FE acquired by the main beam photodetector 121 by calculating the above-described equation (1) is 0.
If the objective lens 115 is too close to the recording surface 105 of the optical disc 104, the focusing error signal FE acquired by the main beam photodetector 121 is positive. If the objective lens 115 is too far from the recording surface 105 of the optical disc 104, the focusing error signal FE is negative.
A tracking error signal TE is acquired, as the side beam photodetectors receive the plus-first-order light and the minus-first-order light split by the diffracting grating 112 and the difference between the outputs of the side beam photodetectors is calculated.
In the optical pickup device having the optical system 101 constituted as described above, the objective lens 115 is driven and displaced on the basis of the focusing error signal FE acquired by the main beam photodetector 121 of the light receiving unit 116 and the tracking error signal TE acquired by the side beam photodetectors. Thus, the objective lens 115 is moved to the focusing position with respect to the recording surface 105 of the optical disc 104 and the light beam is focused on the recording surface 105 of the optical disc 104, thereby reproducing information from the optical disc 104.
In the optical system 101 provided in the above-described optical pickup device, if the center of the beam spot cast onto the light receiving surface of the main beam photodetector 121 is slightly deviated in any direction from the center of the main beam photodetector 121 as shown in FIG. 3 when acquiring the focusing error signal FE by the above-described light receiving unit 116, the output in the case of the accurate focusing state is no longer 0 and an offset is consequently given to the focusing error signal FE.
The optical system 101 has a problem that since the focusing control is carried out so that the focusing error signal FE becomes 0, the objective lens 115 cannot be controlled to be driven to the accurate focusing position.
In the above-described optical pickup device, the center of quadrisecting of the light receiving surface of the main beam photodetector 121 must be situated accurately at the position conjugate with the emission point of the light source 111 in order to acquire an appropriate focusing error signal FE which enable control of the objective lens 115 to an appropriate position.
To secure high position accuracy of the light receiving unit 116 with respect to the light source 111 as described above, the position accuracy of the light receiving surface of the main beam photodetector 121 must be strictly, controlled with respect to, for example, the position standard of a package, when manufacturing the main beam photodetector 121.
Therefore, the above-described optical system 101 is a hindrance to reduction in the manufacturing cost of the light receiving elements such as the main beam photodetector 121 and improvement in the productivity of the assembly process of the optical pickup device. Consequently, it may cause an obstruction to reduction in the manufacturing cost of the optical pickup device itself or may lower the quality of the optical pickup device.